The invention relates to a continuous digester system, and method of treating comminuted cellulosic fibrous material, in order to achieve optimum uniformity of treatment and uniformity of material movement in the digester.
Conventional continuous digesters perform their designed functions quite well, in general. However a continuing practical problem in conventional continuous digesters is uniformity of treatment. Because of the variety of feed material and treatment liquids, and the enormous size of modern continuous digesters, it is not unusual to experience uniformity problems. Some parts of the material column may fail to move properly through the digester with resultant nonuniformity of treatment of the material, and improper liquid flows in some parts of the column also introduce undesirable variations in the treatment of the material. Nonuniformity of the pulp discharged from the digester can, of course, adversely affect subsequent treatment stages, and the quality of the end product produced. Therefore it is highly desirable to effect treatment that is as uniform as possible.
According to the present invention, several radical departures are made vis-a-vis conventional continuous digesters, with a view toward increasing uniformity of treatment. In conventional continuous digesters, feed liquor is separated from the introduced wood chips (or like comminuted cellulosic fibrous material such as bagasse, agricultural wastes, etc.) at the top of the digester, above the level of the chips column formed in the digester. The chips column that does form has a generally conical taper at the top thereof. Treatment liquors that are introduced into the digester move with relatively high velocity through the chips in order to minimize possibility of screen clogging, and every effort is made to minimize channeling since if channeling occurs--especially with relatively high treatment liquor velocities--significant nonuniformity of treatment results.
The digester, and method of treatment of wood chips, according to the present invention differ in every aspect from the conventional practice as described above. For instance according to the present invention, separation of feed liquor from the chips does not occur at the top of the vessel, but rather below the top of the chips column. The chips entrained in feed liquor are introduced substantially horizontally into the top of the vessel, with a swirling action, and result in the establishment of a chip column having a generally horizontal top surface. Such a top surface--compared to the conical top surface that normally is produced in conventional practice--lends itself to much more uniform treatment of the chips, fewer hang ups, and the like. Withdrawal of feed liquid takes place below the top of the chips column, utilizing withdrawal screens. Treatment liquids introduced into the digester are introduced utilizing a vertically extending conduit concentric with the digester which has a relatively large diameter, resulting in the introduced liquids having a relative low velocity. Sets of vertically spaced withdrawal screens in the digester, which provide for recirculation of the treatment liquids and the like, are operated in such a manner that channeling is deliberately introduced into the chip column, the channeling action progressively moving around the circumference of the digester so that it is practiced in circumferentially consecutive radial segments of the digester.
The practice of the method according to the present invention is accomplished by providing a plurality (preferably at least three, and normally more) sets of vertically spaced screen systems in the digester. For each screen system a plurality (preferably at least four, and normally more) liquid withdrawal nozzles are provided circumferentially spaced from each other, with each nozzle cooperating with only a particular radial segment of the screens to facilitate withdrawal of liquid through that radial segment only. The radial segment of the various screen sets are generally vertically aligned, and withdrawal through the nozzles is practiced so that liquid is being withdrawn from the same radial segment of each screen set at the same time. Such withdrawal is preferably accomplished utilizing a plurality of rotary valves operated by a common shaft and operator, each valve being connected to the withdrawal nozzles associated with each screen set.
At the bottom of the digester a discharge mechanism is provided for discharging cooked, quenched, and washed pulp. The discharge mechanism preferably comprises a rotating scraper operatively connected to a rotating outlet, and sluicing liquid is introduced into the bottom of the digester at substantially the same radial segment as the rotating outlet, the sluicing liquid entraining pulp and flushing it through the outlet, so that it is discharged from the digester. The introduction of sluicing liquid, and the rotation of the outlet and scraper, are synchronized with respect to the treatment liquid withdrawal so that discharge of pulp is always taking place at a radial segment of the digester approximately opposite to the radial segment at which liquid withdrawal is taking place. A rotary valve connected to the same shaft as the withdrawal liquid valves preferably provides for the synchronized introduction of the sluicing liquid.
A wide variety of chips treatments may be practiced according to the present invention, but usually the chips would be impregnated, cooked, quenched, and then washed in the continuous digester according to the invention, and in the practice of the method according to the invention.
It is the primary object of the present invention to provide a continuous digester, and a method of treating wood chips (or other comminuted cellulosic fibrous material), that optimize the uniformity of treatment. This and other objects of the invention will become clear from an inspection of the detailed description of the drawings, and from the appended claims.